The present invention relates to a covered microporous stent and method of coating. Coated stents are known as are stents that are designed to expand at the site within a blood vessel where they are intended to engage the internal wall of the blood vessel to reinforce it.
Currently available stents are known to cause restenosis. Restenosis can be caused by incompatibility of the metallic surface of the stent that engages the inner walls of the blood vessel giving rise to subacute thrombosis, usually within four weeks of implantation of the stent. Another possible cause of restenosis is the recoil of the metallic surface of the stent when placed along the internal wall of the artery. Such recoil does not occur immediately, but, rather, when the stent pulses due to its elastic nature, stresses act downwardly toward the stent resulting in gradual diminishing of the luminal diameter.
Additionally, most commercially available stents have an open mesh area when they are expanded and rapid ingrowth of collagen cells and smooth muscle cells gradually occurs through these open areas. Collagen cells contributing toward intimal hyperplasia eventually result in the narrowing of the lumen diameter. Additionally, sometimes, at the location where the stent is to be placed, such as, for example, in saphenous vein grafts, thrombus exists that must be bypassed. Finally, in the aorta and peripheral arteries, a large portion of the aneurysmal sac may have to be bypassed prior to stent implantation.
Use of polymeric or biologic material to coat a stent is generally known. However, in such coated stents, when they are expanded within a blood vessel, the coefficient of expansion of the coating greatly differs from that of the expanding stent such that, upon expansion, the surface of the coating tears rendering the expanded stent uncoated. If, as a solution, the coating is applied more thickly, such a thick coating can deter stent expansion or can have such a high profile that implantation into the body is difficult. The same problem occurs when a stent is coated by sandwiching it between inner and outer coating layers. If the stent cannot be expanded to its full intended diameter, the success of the implantation can be drastically compromised.
While the prior art does describe the concept of providing a stent with a coating, Applicant is unaware of any prior art that specifically describes whether such a coating surrounds the individual members of the metallic stent or whether such a coating covers all of the gaps between the various interstices of the expanded stent.
The prior art also describes biologic coatings such as collagen or gelatin to be employed on a balloon expandable or thermally expandable stent. To coat such a stent, it is placed in a mandrel and then dipped into the biologic solution and then dried in an oven.
Applicant is also aware that stents have been coated with a polymeric material such as silicone that renders the resulting stent impervious to any incursion of biologic tissue. However, in such stents, the impervious nature of the coating deters the healing of the surrounding tissues.
The following prior art is known to Applicant:
U.S. Pat. No. 5,234,457 to Andersen PA0 U.S. Pat. No. 5,330,500 to Song PA0 U.S. Pat. No. 5,344,426 to Lau et al. PA0 U.S. Pat. No. 5,443,499 to Schmitt.
None of these patents teaches the features and aspects of the present invention as set forth hereinbelow, including the particular stent coatings nor the method of so coating a stent.